This invention relates to a method for controlling the idling operation of a spark-ignited internal combustion engine, and particularly to such methods as make use of ignition pulse timing control in order to stabilize the engine idle speed.
German Patent Disclosure No. 2,221,354 discloses a method wherein engine spark-ignition timing is controlled to provide stabilization of the idle speed of an internal combustion engine. The disclosure is aimed particularly at engines wherein a change in engine load, which may result from the use of hydraulic power steering, causes a reduction in idle speed. In order to compensate for the increased load on the engine, the ignition timing of the engine is advanced to provide increased engine torque and increased engine speed.
This prior art ignition timing control method and apparatus makes use of the relationship between engine speed and ignition timing set forth in FIG. 1. FIG. 1 is a plot of engine speed n, relative to maximum speed n.sub.max available with constant air ratio and air volume throughput, as a function of ignition timing advancement .alpha..sub.z relative to top dead center. By reference to FIG. 1 it may be seen that engine idle speed n is approximately linearly related to ignition advancement .alpha..sub.z such that modification of the ignition angle .alpha..sub.z in the direction toward advance ignition (positive angles) will result in an increase in idle speed n over a relatively large angular variation up to a position of approximately 40.degree. advance relative to top dead center, at which point a maximum engine speed n.sub.max will occur for the given air-fuel ratio .lambda. and air throughput Q.sub.L.
FIG. 2 illustrates the relation between mean indicated pressure Pmi and ignition timing advancement .alpha..sub.z and indicates that approximately the same relationship is present between ignition timing advancement and mean indicated pressure, which is an indication of engine torque. Consequently, with increased ignition timing advancement .alpha..sub.z the engine torque output will also increase to a maximum.
The relationships of FIGS. 1 and 2 are used in accordance with the mentioned German patent disclosure for stabilization of the idling speed of an engine with variations in the load on the engine. In accordance with the method, the engine is forced to idle with considerably retarded ignition and the resulting output loss of the engine, on account of retarded ignition, is compensated for by an enlargement of the fuel and air charge supplied to the engine combustion chambers.
While the previously known arrangement for stabilizing idle speed is directed at variations in applied engine load, the purpose of the present invention is to provide a method for operating a spark-ignited internal combustion engine with a lean fuel-air mixture during engine idling. Such a mixture is desirable for reducing engine exhaust gas emissions, particularly carbon monoxide and hydrocarbons. Operation with such a lean fuel-air mixture can often result in variations of the air mixture, both time variations of the mixture supplied to one of the cylinders, and variation in the mixture supplied to various cylinders. Such variations in the fuel-air ratio supplied to the cylinders of the engine can result in rough engine operation and possibly even stalling of the engine during idling with such a lean mixture.
It is therefore an object of the present invention to provide a new and improved method for controlling the idling operation of a spark-ignited internal combustion engine.
It is a further object of the invention to provide such a method which reduces the pollutant emissions of the engine.
It is a further object of the invention to provide an apparatus for controlling the ignition timing of an internal combustion engine in accordance with the method of the invention.